Yes I did test each stick individually and no errors came back. My motherboard mfg suggests when only using one stick to use a specific DIMM only (the furthest right slot) - will it cause problems if I use only one stick in other slots?
Also tested just 2 sticks in and no errors. Now I have 4 sticks back in and it's on test 10 in the first pass, no errors so far.
The only guess I have is that somehow something came loose, although I haven't moved the tower. I did actually check to make sure each stick was in securely before I started any testing at all, and everything seemed fine. I'm not sure if anything else could cause results like this.
I should also mention that the first ram kit I had actually did have a bad stick, so it was replaced with this set that seemed to work well, for a while at least, after I installed it. I was feeling really annoyed that even the replacement ram potentially has problems...